The emergence of WNV in the US in 1999 and demonstration of transfusion-transmission (TT) in 2002 alerted the global blood banking community of the blood safety implications of arbovirus epidemics. Since 2002, investigators in this proposal have conducted systematic research on WNV infected donors and exposed recipients and guided donor RNA (NAT) screening and deferral policies that have virtually eliminated TT-WNV. We also took advantage of unparalleled access to WNV RNA+ donors to capture individuals in the pre- symptomatic stage of infection. Our systematic longitudinal studies of viremic donors have contributed to understanding of the natural history of WNV diseases, and elucidated complex interactions of viral and host genetic and innate and adaptive immune responses that determine progression to symptomatic WNV disease. Further work is needed, however, to establish the infectivity of low-level viremic units in the early convalescent stage of infection not detected by current NAT screening, and additional pathogenesis questions remain to be addressed through ongoing enrollment and follow-up and collaborative study of viremic donors. Dengue is the most important arbovirus in the world, with 50-100 million infections and >25,000 deaths annually in tropical/semi-tropical regions of the world. After decades of absence of in the continental US, clusters of dengue cases have been documented in the southern US over the past several years, and expanded spread is now a real possibility. Although healthcare transmissions have been difficult to ascertain in endemic countries, two clusters of TT-DENV were recently reported, and we have documented high rates of viremia in blood donors in Central/South America and Puerto Rico (PR), a US territory whose blood supply is run by the American Red Cross. Although DENV is among the highest priority risks to blood safety, routine screening of donors is not currently conducted in epidemic regions due to lack of systematic data on viral dynamics and infectivity of acute viremia that is needed to drive development/licensure of tests and formulation of donor screening guidelines. The investigators in this proposal will implement sensitive NAT screening in PR under an FDA IND, and launch follow-up studies of DENV+ donors similar to those conducted on WNV infected donors. By defining viral and immune dynamics and the duration of infectivity in primary and secondary infections by different DENV serotypes, we will accelerate implementation of appropriate DENV donor screening and advance our understanding of DENV diagnostics and immunopathogenesis. A comprehensive repository will also be established of the pedigreed samples from donors identified in the acute viremia stages of WNV and DENV infections, and then followed through viral clearance and development of symptomatic infection. This repository of plasma and PBMC specimens and databases with linked clinical and laboratory data will be transferred to the NHLBI Specimen Repository so that these unique samples are accessible to qualified scientists studying blood safety and the pathogenesis of WNV and DENV. (End of Abstract)